Water transport through hydrocarbon-based proton exchange membranes

Understanding the hydrocarbon - PFSA ionomer conductivity gap in hydrogen fuel cells

Hydrocarbon ionomers (HCs) have the potential to replace perfluorinated sulfonic acids (PFSAs), which are currently used in electrolyser or fuel cell membranes. To be a truly viable alternative, HCs must have conductivity across the operating range and cell lifetime comparable to PFSAs. Conductivity is an important property of membranes because it affects the energy efficiency of a fuel cell or electrolyser. By examining conductivity as a function of water volume fraction, it becomes evident that HC ionomers have consistently lower conductivity at low relative humidity. To better understand this 'conductivity gap', conductivity was converted to proton diffusivity and analysed using General Effective Media (GEM) theory for the first time. This analysis revealed that all ionomers require similar hydration levels for proton dissociation, and proton diffusion coefficients in the dry polymer are responsible for the conductivity gap. It is suggested that the membrane tortuosity ultimately accounts for the dry membrane's proton diffusivity and low RH conductivity. As the membrane hydrates however, all ionomers exhibit similar diffusion coefficients, indicating that conductivity at high humidity is limited by proton concentration.

Isopropanol Modified Hydrocarbon-Based Polymer: Toward an Environmentally Friendly Large-Deformation Soft Actuator

Ionic polymer-metal composites (IPMCs) with deformability are proposed as promising candidates for artificial muscles. However, the deficiencies of their most commonly used perfluoro polymer (Nafion) substrate cause non-negligible restrictions on its development. In this study, a novel environmentally friendly hydrocarbon-based IPMC is fabricated as an alternative to Nafion-based IPMC and successfully exhibits superior electrochemical characteristics (strip resistance reduced by 46% and capacitance increased 13-fold) and deformation performance (tip displacement of 41 mm at 3 V). Given its merits of fluorine-free, low cost (1/20 of Nafion), and no significant back relaxation, the hydrocarbon-based polymer is anticipated to be a possible solution to overcome the inherent drawbacks of perfluorinated substrates. Additionally, a series of multiform ultralow voltage (≤2.5 V) biomimetic flexible grippers are first designed using hydrocarbon-based IPMCs and show potential functionalities for capturing, orientating, and ejecting in fields such as biomimetic robotics, narrow-space engineering tasks, and design of miniature gripping devices.

Measurement of Water Uptake and States in Nafion Membranes Using Humidity-Controlled Terahertz Time-Domain Spectroscopy

Perfluorinated sulfonic acid ionomers are well known for their unique water uptake properties and chemical/mechanical stability. Understanding their performance-stability trade-offs is key to realizing membranes with optimal properties. Terahertz time-domain spectroscopy has been demonstrated to resolve water states inside industrially relevant membranes, producing qualitatively agreeable results to conventional gravimetric analysis and prior demonstrations. Using the proposed humidity-controlled terahertz time-domain spectroscopy, here we quantify this detailed water information inside commercially available Nafion membranes at various humidities for direct comparison against literature values from dynamic vapor sorption, differential scanning calorimetry, and Fourier transform infrared spectroscopy on selected samples. Using this technique therefore opens up opportunities for rapid future parameter space investigation for membrane optimization.